Robert Rissland, Director, Int’l Sales for Micro Pulse LiDAR, part of Hexagon, explains the advantages of the polarized, single-photon detecting Mini Micro Pulse Lidar (MiniMPL) over Doppler Wind Lidar systems.
For wide-area aerosol detection, differentiation and concentration mapping, the best tools for the job must offer autonomous continuous operations in varying weather conditions and collect reliable data to support accurate analysis. When considering the critical performance differences between polarized aerosol lidar such as Micro Pulse LiDAR (MPL) and Doppler Wind LiDAR, it is clear that MPL is the only realistic remote sensing technology for mapping aerosols. This type of polarized technology is the basis for the MiniMPL instrument, a rugged and portable scanning unit optimized for a variety of applications related to emissions monitoring and air quality.
One of the unique features of the MiniMPL is its coverage range. With a vertical range of 15 km and a horizontal range of 6 km, data are collected over a much broader area than with most Doppler LiDARs, except for the biggest most expensive systems on the market. The scan patterns of the MiniMPL are also fully programmable to suit the individual application. If the mission is to provide wide-area coverage for tracking and analysis of dust plumes or other emissions, the MiniMPL is the most cost-effective solution.
Even more significant, the MiniMPL data set supports important capabilities, such as estimating the height of the Planetary Boundary Layer (PBL), measuring mass concentration of all aerosols in the column down to PM0.532, and differentiating between dust, pollen and water particles in the atmosphere. This level of detail is made possible by its highly sensitive photon-counting detectors and the continuous collection of co-polarized and cross-polarized backscattered light. The ratio of these polarizations is used to classify scatter from biogenic aerosols, like pollen, versus other particles, because each shape interacts with light in a different manner. Doppler LiDAR does not have these capabilities and thus cannot distinguish an aerosol water cloud from an aerosol dust cloud.
Also, the MiniMPL produces more accurate localized results because it is a “backscatter” line of sight instrument with a narrow field of view, meaning it does not calculate averages between profiles in the way that Doppler LiDAR does. Upward wind direction is perpendicular to the generally horizontal wind, which means that the radial wind velocity component is very small. Doppler devices overcome this by scanning the atmosphere in a large angle to get a reasonable radial component. This large scan angle means the results are averaged over large distances, over which the wind is unlikely to be constant. This makes Doppler devices inherently deficient in complex terrain (like mines) where wind changes noticeably over short time and distance scales. This deficiency is not a factor for the backscatter lidar.
Another important consideration is system reliability. Doppler technology is by nature more complicated, so the instruments have more failure-prone parts that require frequent servicing and are more costly to acquire, operate, maintain and repair over the life of the instrument. Over the long run, the more complex Doppler systems will have more downtime for service than the MiniMPL. Imagine the service time and cost of using a Ferrari as your daily transportation, versus a BMW or Audi. The MiniMPL sensor suite consists of robust instruments that have been tried and tested around the world in varying harsh conditions. For a lower upfront cost compared to typical Doppler LiDAR instruments, the MiniMPL requires far less routine maintenance and is less expensive to operate, while providing higher uptime and reliability.
MPL technology is effective for all kinds of aerosol monitoring applications, e.g., tracking dust at open pit mines, monitoring air quality in urban areas, identifying volcanic ash, and detecting ice clouds. In contrast, Doppler LiDAR looks at general aerosol content of the atmosphere and uses observations to calculate wind speed and direction. The Doppler technology is unable to reliably discriminate between aerosols by species, size and concentration. Doppler LiDAR cannot provide any insight into the mass loading of the aerosol or even support emissions flux calculations. Only the polarized backscatter MPL can collect data necessary to perform calculations such as PBL height and mass concentration of all aerosols and distinguish between aerosol types.
Mr. Rissland has over 40 years of international business development experience in the meteorological and environmental markets, with a special focus on active and passive remote sensing technologies. For more information about why Micro Pulse LiDAR is the most effective technology available for aerosol mapping, please contact Bob at Robert.Rissland@micropulselidar.com.